Flash memory devices typically can maintain stored information regardless of power supply and can be classified into devices having a NOR structure and devices having a NAND structure, which are different configurations for connecting memory cells to a bit line and a source line.
FIG. 1 is a block diagram showing a memory cell array 100, along with an X-decoder 110 and a Y-decoder 120, which are peripheral circuits of the memory cell array 100, in a conventional NAND flash memory device. FIG. 2 is a circuit diagram illustrating a structure of the memory cell array 100.
Referring to FIGS. 1 and 2, the NAND flash memory device includes a memory cell array 100 including a plurality of memory cell blocks 100A, each comprising a plurality of memory cells. The X-decoder 110 selects word lines WL0, WL1, through to WLm−1, and WLm of the memory cell blocks 100A and the Y-decoder 120 selects bit lines BL0, BL1, through to BLn−1 and BLn of the memory cell blocks 100A. A Y gating circuit 130 is connected to the Y-decoder 120 to designate a bit line path in the memory cell array 100.
Referring to FIG. 2, the memory cell blocks 100A of the memory cell array 100 include a plurality of cell strings 10 formed between the bit lines BL0, BL1, . . . , BLn−1, BLn and a common source line CSL. Each cell string 10 includes a plurality of memory cells 12 connected in series. Gate electrodes of the memory cells 12 included in one cell string 10 are connected to respective word lines WL0, WL1, . . . , WLm−1, WLm. A ground selection transistor 14 connected to a ground selection line GSL and a string selection transistor 16 connected to a string selection line SSL are connected in series with the memory cells 12 at respective ends of the cell string 10. The ground selection transistor 14 and the string selection transistor 16 control electrical connections between the memory cells 12 and the bit lines BL0, BL1, . . . , BLn−1, BLn, and the common source line CSL. Memory cells 12 connected to one of the word line WL0, WL1, . . . , WLm−1, WLm across the cell strings 10 form a page unit or a byte unit.
In the NAND flash memory device of FIGS. 1 and 2, in order to perform a read operation or a write operation, a memory cell is read by selecting one of the word lines WL0, WL1, . . . , WLm−1, WLm and one of the bit lines BL0, BL1, . . . , BLn−1, BLn using the X-decoder 110 and the Y-decoder 120, respectively.
Typically, a NAND flash memory device has a relatively high integration density. However, further reduction of the design rule of NAND flash memory devices is desired to further reduce chip size. As the design rule is reduced, minimum pitch of patterns required for constituting the NAND flash memory device may be greatly reduced.
In order to realize a minute pattern that meets the reduced design rule, various methods of forming patterns may be employed. For example, in order to realize a cell array structure of NAND flash memory device that is difficult to realize using current photolithography techniques, a double patterning technique for forming repeated patterns with a pitch less than the limits of conventional lithography techniques has been developed. When a NAND flash memory device is manufactured using such a double patterning technique, for example, when the word lines WL0, WL1, . . . , WLm−1, WLm are formed, the ground selection line GSL and the string selection line SSL may be simultaneously formed.
In a conventional NAND flash memory device, a contact pad for connecting the word lines WL0, WL1, . . . , WLm−1, WLm to the X-decoder 110 may be integrally formed with the word lines WL0, WL1, . . . , WLm−1, WLm. The contact pad connected to the word lines WL0, WL1, . . . , WLm−1, WLm may be simultaneously formed with the word lines WL0, WL1, . . . , WLm−1, WLm. Thus, when the word lines WL0, WL1, . . . , WLm−1, WLm are formed using the double patterning technique, a trimming process for removing undesired portions of minute patterns formed around the contact pad for connecting to the peripheral circuit may also need to be performed. The same trimming process may also be applied when a contact pad for connecting the bit lines BL0, BL1, . . . , BLn−1, BLn to the Y-decoder 120 is integrally formed with the bit lines BL0, BL1, . . . , BLn−1, BLn.
However, in some conventional NAND flash memory devices, the structure of contact pads connected to word lines and bit lines is minute and complicated, and thus, a layout of a mask pattern for the trimming process may be complicated. In particular, design rules for NAND flash memory devices have been greatly reduced according to recent market demand, and the pattern sizes of word lines and bit lines in NAND flash memory devices are generally becoming more minute. Accordingly, the structure of the contact pads for connecting peripheral circuits and the word lines and the bit lines have generally become even more minute and complicated. Therefore, the layout of a mask pattern for that trimming process may also be minute and complicated.
Also, because the pitch between the minute patterns formed by the double patterning technique may be very small, when a mask pattern for trimming is formed, a tolerance of an alignment error between the minute patterns formed using the double patterning technique and the mask pattern may need to be very strict. Thus, due to a possibility of misalignment occurring during performance of an aligning process and variations in the etching process, patterns may be unintentionally removed or an undesired pattern can be obtained when the trimming process is performed.